While the optical axis of the eye is characterized by the straight line between the centers of curvature of refractive surfaces, the axis that extends from the “fovea centralis” through the nodal point of the eye to the fixing object is designated as the visual axis. If the different media are mathematically reduced to a single medium with an average index of refraction and a spherical curvature, a point in the eye can be defined through which all beams pass without being refracted. This point is designated as nodal point of the visual axes.
Usually, in all eyes, the visual axis deviates from the optical axis. This results, on the one hand, from aberrations of the eye which, for example, are a result of the fact that curvature radii of the individual eye media are not uniform, the eye lens is tilted, the retina is not in the focus of the eye lens, and much more. On the other hand, when aligning the eye with an object, it is hereby attempted to image said object, whenever possible, in the fovea as the area of the sharpest vision.
While the alignment of the eye plays no role for many examinations in ophthalmology, at least the knowledge about the alignment of the eye relative to the ophthalmological device is absolutely necessary not only for the treatment of the eye but also for the measurement of the eye.
Determining different biometric parameters of an eye is necessary, in particular, prior to a surgical intervention for replacing the eye lens in the event of lens opacity (cataract). In order to ensure optimal vision after the surgery, it is necessary to determine these parameters with adequately high accuracy so as to be able to subsequently select a suitable replacement lens based on the determined measurements. The most important parameters to be determined are, among other things, the axial length (distance from the cornea to the retina), the corneal curvature and corneal refractive power, and the length of the anterior chamber (distance from the cornea to the eye lens).
Thus, for carrying out biometric measurements on the eye it is advantageous if the optical axis of the ophthalmological measuring arrangement and the optical axis of the eye to be measured are aligned with each other and ideally coincide. This ensures during biometric measurements according to the principle of the short-coherence interferometry that from the weak light portions reflected by the boundary surfaces of cornea and lens, sufficient signal intensity reaches the detector and generates a measurable interference contrast.
According to the known prior art, different solutions for interferometric determination of distances in the eye are known, each solution placing different high demands on the alignment of the eye with the ophthalmological device.
A first arrangement for interferometrically measuring the distances of the anterior eye segments is described by Drexler, W. et al. in [1]. For aligning the visual axis of the eye with the optical axis of the measuring arrangement, collimated fixing light is reflected along a fixed coaxial direction into the measuring beam path. Adjusting the angle between the visual axis of the patient and the optical axis of the measuring arrangement is carried out with the aid of a scanning mirror. For this, the alignment of the two axes has to be performed with an accuracy better than 1° because otherwise no overlapping of cornea reflex and lens reflex occurs on the detector and no analyzable interference signal arises. Accordingly, the sensitivity of the setup to tilting of the patient's eye is very high. Here, the position of the visual axis of the eye is determined by means of scanning over a predetermined angular range in two orthogonal spatial directions. The method described here is very time-consuming and, in addition, is not reliable enough in daily clinical practice.
Another solution for determining distances at the anterior eye segment, preferably the pupil and/or iris diameter, is described in DE 101 08 797 A1. Here too, the proband is offered a light mark on which he/she fixes his/her eye. During the entire time of adjusting and measuring, the user can visually monitor that the proband fixes correctly. In contrast to the schematic eye according to Gullstrand, in real eyes, the visual axis and the optical axis can deviate from each other by up to 8° since the fovea can be offset from 3° nasally to 8° temporally. Accordingly, it can be advantageous here to offer the proband an offset fixing light. As in the previously described solution, the alignment of both eyes with each other has to be carried out with high accuracy. This is of particular importance here since during the determination of pupil and/or iris diameters, not scattered light but reflected light is detected.
A significantly improved method is described in U.S. Pat. No. 7,380,939. Here, instead of a cost-intense scanning mirror, fixing light generated by an LC display is used. Said fixing light is not only variably adjustable in terms of its lateral position but also in terms of its apparent distance from the patient's eye so as to take account of a potential defective vision of the patient.
In addition, through intentionally defocussing the measuring beam by means of axial displacement of the focusing lens relative to the patient's eye, cornea reflexes and lens reflexes are imaged on the detector in an unsharp and enlarged manner. Through this, the sensitivity of the measuring arrangement to tilting or lateral displacement of the patient's eye is slightly reduced.
However, the disadvantage is that due to the use of a dual beam interferometer, the measuring light portions reflected by different boundary surfaces in the eye generate the interference signal; thus, no external reference light serves for generating the interference contrast. For this reason, only non-diffraction-limited imaging of the reflected light on the photo detector can be selected so that only specular (Fresnel) reflexes contribute to the interference signal. The speckles caused by volume scattered light are significantly smaller in the detector plane than the detector area and are therefore “averaged out” over the detector area. As a result of this, the measurement method continues to be dependent on a generally very precise pre-adjustment to the visual axis of the patient's eye.
A third technical solution known from the documents WO 2001/38820A1 and WO 0207/053971A1 uses a diffraction-limited capture of the measuring light scattered back in the eye for interferometrically determining distances in the eye; however, this solution has only one fixing light fixedly positioned on the optical axis of the measuring arrangement. Thus, with this arrangement, a variable fixing of the viewing direction, which is advantageous for the measurement, cannot be achieved.
Literature:
    [1] Drexler, W. et al.; “Submicrometer Precision Biometry of the anterior Segment of the Human Eye”; Investigative Ophthalmology & Visual Science, Vol. 38, No. 7, pages 1034ff